In one type of nuclear reactor being developed, i.e., simplified boiling water reactor (SBWR), various pools of water are provided in the containment building surrounding a reactor pressure vessel which provide various functions, including providing makeup water to the pressure vessel in the event of a postulated loss of coolant accident (LOCA). The pressure vessel contains a nuclear reactor core which is effective for boiling water therein to generate steam under pressure which is conventionally discharged from the pressure vessel to provide a source of power to a steam turbine-generator, for example, for producing electrical power. In the event of a break in one of the several pipes joined to the pressure vessel, water and steam will leak from the pressure vessel, which will drop the level of water therein available for cooling the reactor core unless suitable provisions are made to provide makeup water into the pressure vessel.
For example, a conventional boiling water reactor power plant includes a wetwell or suppression pool of water in the containment building surrounding the pressure vessel. The suppression pool provides various functions during the operation of the power plant, including, for example, being joined in flow communication to the pressure vessel by a suitable conduit extending from the suppression pool to a corresponding inlet nozzle joined to the pressure vessel. In the event of a LOCA condition, a suitable valve is opened for allowing water to flow by gravity from the suppression pool into the pressure vessel to provide makeup water therein.
In order to provide additional makeup water during a LOCA condition, the SBWR design further includes a gravity driven cooling system (GDCS) which has a corresponding GDCS pool of water elevated above the suppression pool which is similarly joined to a corresponding inlet nozzle on the pressure vessel for selectively providing makeup water thereto upon opening of a suitable valve in the supply pipe therebetween. In both the GDCS pool and the suppression pool, it is desirable to have makeup water provided to the pressure vessel solely by gravity flow to avoid reliance on power operated pumps which could be rendered inoperative in the event of a power failure.
However, since water supply lines must necessarily be provided between the pressure vessel and both the GDCS pool and the suppression pool, a break in one of these lines would also cause a loss of coolant accident as well as possibly disable their use for providing makeup water to the pressure vessel.
Accordingly, it is desired to join inlet nozzles on the pressure vessel to the suppression pool and the GDCS pool with supply lines having minimum resistance to the gravity flow of makeup water from these pools in a forward flow direction into the pressure vessel in the event of a LOCA condition, but, a competing consideration requires relatively high flow resistance in the event of a break of one of these supply lines for restricting back flow of water from the reactor through these inlet nozzles.